1. Technical Field
This invention relates generally to inertial balances used to measure mass and more particularly, to an active force compensator for an inertial balance which improves the stability and measurement accuracy of such instruments.
2. Background Art
Inertial balances measure mass inertially and can therefore operate independent of gravity and in any orientation. Such instruments are used in a wide variety of technically demanding industrial and research applications in areas ranging from coal mines to outer space.
Generally, an inertial mass measurement instrument consists of a mass-spring system where the mass to be measured is put into oscillation while being supported by an elastic element having a known spring rate. When the mass to be measured changes magnitude, the natural or resonant frequency of the system changes. This frequency change is measured and then used to calculate the mass change. However, when the elastic element is not affixed securely to the earth, the mass, spring constant and damping properties of support structures can influence the resonant frequency of the system and result in erroneous mass readings. The purpose of this invention is to eliminate such erroneous readings.
In an ideal inertial mass measurement system in which the elastic element supporting the mass to be measured is affixed to a stable inertial reference, the resonant frequency of the system is uniquely determined by the equation: EQU f.sup.2 =(1/4Pi.sup.2) (Kg/W) (1-C.sup.2),
where f is the resonant frequency, K is the spring constant, g is the gravitational constant, W is the weight of the oscillating mass, and C is the fraction of critical damping contributed by the internal and external damping of the elastic element and oscillating mass.
Real systems can only approximate the stable inertial reference on which the above equation relies for accuracy. Real systems have no firm connection to the earth and thus have far more complex equations which yield two or more resonant frequencies. To uniquely determine the primary resonant frequency associated with the motion of the elastic element and oscillating mass, these complex equations, which include terms or factors for each mass, spring and damping property in the supporting structure, have to be taken into account. If the value of any of the properties of any of the supporting structure elements change, the primary resonant frequency also changes. Usually the inertial instrument manufacturer does not have control over secondary support elements, which include the tables, shelves or other supports employed by the user. Such support elements are subject to property changes through temperature variations, aging and other environmental effects. These property changes result in spurious frequency changes that are not related to mass loading.
In the past, passive suspensions and passive compensators have been employed in attempts to limit the effects of these property changes to a tolerable level. However, none were capable of eliminating the effects entirely.
U.S. Pat. No. 3,926,271 describes an extremely sensitive inertial microbalance capable of measuring the mass of very fine particles and other matter. This instrument employs a tapered elongate elastic element having a first end which supports matter to be measured and a second larger end which is anchored to a primary support so that the first end and matter carried thereby are free to oscillate. The elongate element is excited into oscillation at a resonant frequency. The resonant frequency of the oscillating element varies in accordance with the mass loading and accordingly can be monitored and measured to determine the mass of matter supported by the oscillating element. An improvement which facilitates use of this microbalance for the measurement of the mass of particulate or other forms of matter contained within a medium such as air or other fluids is described in U.S. Pat. No. 4,391,338. The contents of these two patents are incorporated by reference herein.
In practice, the oscillating tapered element microbalance has proven to be a valuable instrument which permits on-line, real-time direct measurement of particulate mass with great sensitivity and reliability. The instrument has been successfully employed in the evaluation of diesel exhaust, dust concentration and smoke measurement, and is applicable to many other situations in which particles or other extremely fine forms of matter need to be detected and weighed.
The tapered element in the above-described microbalance vibrates in a clamped/free mode. At the clamped end, where the tapered element meets the housing or primary support, energy flows out from the oscillating tapered element through the housing into the secondary support structure. As generally described above, this transfer and dissipation of energy into the secondary support structure introduces a frequency uncertainty which can affect the accuracy of the microbalance.
One approach for coping with this problem is to attach the housing of the microbalance to a larger rigid distributed mass which is then decoupled by standard means such as a foam rubber cushion from the environment. However, this approach is rather cumbersome and not suitable for certain applications.
In U.S. Pat. No. 4,696,181 a DECOUPLING SUSPENSION SYSTEM FOR AN OSCILLATING ELEMENT MICROBALANCE is described which employs longitudinally extending suspension members axially aligned with an instantaneous center of rotation of the microbalance. The suspension members store energy transferred out of the oscillating element through the housing and then return this energy to the housing. This passive mechanical suspension system thus isolates motion of the balance from the secondary support structure and external environment but causes the housing to rock, posing difficulties in attaching hoses or other auxiliary equipment to the moving housing.
An improvement in the measurement accuracy of inertial mass balances is provided, according to the teachings of U.S. Pat. No. 4,838,371, by a suspension system which constrains an oscillating weighing platform to undergo uniform linear motion. This suspension system includes a primary support framework and a plurality of elongate elastic supporting members for suspending the weighing platform from the support framework. In one embodiment, counterweights are attached to extensions of the supporting members. The counterweights are selected and employed to compensate for displacement of the center of mass of the moving portion of the inertial balance. As a result, during vibration, the center of mass of the entire system stays fixed with respect to the support framework thereby preventing force from being relayed through the framework to the outside world and minimizing damping losses to the outside. Although this mechanical approach provides a first order passive compensation for inertial forces, it doesn't permit automatic adjustment with mass loading and is not universally applicable to all inertial mass balances.
A need therefore persists for a compensator for inertial mass measuring instruments which enables accurate frequency readings to be continually taken free from the influence of property changes in the secondary support structure.